An Israel/Iran war could bring in the US and the UN on the side of Israel and Russia/the other middle eastern countries that hate Israel (95% of them) on the side of Iran.

First of all, World War would be hard to define. It would require a massive amount of global participation.
Second, WW2 happened because Germany and Hitler actually had the manpower and the resolve to take over the fucking world. There isn't a country like that now.
Third, let's say a rogue state like North Korea or Iran actually nukes their respective rivals (South Korea and Israel). If that happened, they would be squashed immediately. Hence, nuclear warfare, but not a World War, per se.
Fourth, let's say someone actually has the brass cajones and know-how to slip a nuke over International Waters and detonates it somewhere in the US. Again, they would be destroyed pretty fucking quickly but it wuldn't degenerate into a nuclear war.
As far as I can tell, there are only a few possible scenarios that would bring about WW3. One is if the US and Russia somehow engage in a hot war. This, however, is extremely unlikely. Being that they are the two superpowers (China isn't...yet) they understand that with great power, comes great responsibility.
Another scenario that is WAY outta left field but I suppose is still technically possible is if North Korea and Iran and any other little rogue nations decide to fuck it all and bomb the shit out of everyone around them.
But anyway, excluding anything wildy unpredictable, I don't think a World War will ever happen again. There will always be small wars and cold wars, yes, but not an all-out global nuclear massacre.
Just my 2 cents.